1. Field of the Invention
The invention is drawn to a novel method to produce hydrocarbon and hydrogen fuels simultaneously from biomass by a combination of fermentation and electrolysis.
2. Description of the Prior Art
The essential role of renewable fuels in fostering economically and environmentally sustainable growth is now widely recognized. Although technologies for providing electricity via wind, hydropower, and direct biomass consumption are widely employed, the production of liquid transportation fuels remains the greatest energy security issue in the U.S. today. Corn ethanol, once touted as a major future source of motor fuels, has succeeded in the marketplace only through substantial government subsidies to producers, and questions regarding its environmental sustainability, net energy balance, and role in exacerbating food shortages have now come to the fore.
Cellulosic ethanol is considered to be a more promising long-term source of transportation fuels (Lynd et al., 2002). Cellulosic materials are available in much larger quantities; can be produced on more marginal lands; feature a much larger net energy balance; and do not have a competing human food use. However, despite major funding efforts, development of economically viable cellulosic ethanol technologies have not yet attained commercial success. Chemical pretreatment is considered necessary to enhance the accessibility of the feedstock to enzymatic attack, yet such pretreatment adds costs, generates a waste stream, and produces certain chemical products that inhibit sugar fermentation. Contaminating microbes such as lactic acid bacteria can convert considerable amounts of the hydrolyzed sugars to products other than ethanol, necessitating expensive control measures to permit maintenance of the fermentative monoculture; this is already a major problem in the corn ethanol industry, which has become one of the major users of antibiotics in the U.S. (Olmstead, 2009). Moreover, the most active of the ethanol producers ferment only the hexose fraction of carbohydrate, and even the best strains that utilize the pentose sugar fraction only ferment the carbohydrate fraction but not the other components (proteins, nucleic acids, lipids, organic acids and other phytochemicals) that represent a substantial proportion of plant biomass. This greatly reduces the yield of fuel product. Finally, no obvious use has emerged for the unfermented residue—a critical shortcoming given its likely large volume and the likely low profitability of the cellulosic ethanol process.
Ethanol has a relatively low energy density (with attendant reductions in vehicle miles-per-gallon), and for most gasoline-powered vehicles can only be blended to a low proportion of the total fuel mixture. These disadvantages, along with the slow pace of development of cellulosic ethanol technology, have stimulated a search for routes to convert biomass to hydrocarbon fuels (Regalbuto, 2009). Several such schemes have been proposed. Some of these rely on chemical conversion of biomass materials under heat and pressure (often in the presence of expensive catalysts) and yield either liquids (e.g., pyrolytic oils) or gases that can be reformulated into liquid motor fuels. Several biologically based processes have also been proposed. These processes face some formidable hurdles. For example, photosynthetic algal-based processes either require large areas for cultivation (because of the shallow depth of the photic layer under intense cultivation, while the “dark algal” process or processes based on bacteria that have been genetically engineered for hydrocarbon biosynthesis require sugars as the feedstock (which revisits the high cost of cellulolytic enzymes that has hampered ethanol production via simultaneous saccharification and fermentation).
Biorefinery Processes:
Biorefinery processes that produce various types of biobased fuels from biomass are well known. It is known, for example, that natural mixtures of anaerobic microbial cultures that work together to digest biomass material occur in habitats such as the rumen of ruminant animals, sewage sludge, soil, landfills, aquatic (freshwater, marine, and brackish) sediments, and insect (e.g., termite) guts. These mixed microbial cultures work in concert to provide the necessary enzymes to convert biomass into organic acids. The organic acids are primarily “Volatile Fatty Acids” (VFA) which includes straight and branched chain fatty acids with carbon chain lengths from C2 to C6.
As a result of thousands of years of natural evolution of biomass processing, the ruminal fermentation is a particularly attractive process because it is natural, rapid, and efficient (Hungate, 1950, 1966); it converts most biomass components to useable products (Weimer et al., 2009); and it can readily be conducted in a biorefinery (i.e., in vitro in bioreactors; Goering and Van Soest, 1970).
Ruminal microbes have long been known to convert cellulosic and other feed materials to VFA (Hungate, 1950, 1966), and have also been used for treating organic wastes. In the RUDAD (Rumen-Derived Anaerobic Digestion) process (Zwart et al., 1988), mixed ruminal microbes (including both bacteria and protozoa) are used in a primary stage fermentation to convert cellulosic and other organic wastes to VFA that are passed to a second reactor in which the VFA are converted by other microorganisms to methane and carbon dioxide. The process is used exclusively for waste treatment, although as in many other wastewater treatment plants, the methane produced can be used as a fuel to offset the operating energy requirement of the treatment plant. Differences in the growth rates of microbes in the two reactors, along with problems in maintaining flocculation in the bioreactors, have limited the utility of the RUDAD process (Hack and Vellinga, 1995). An improved process (described in Hack and Vellinga, 1995, U.S. Pat. No. 5,431,819) employs a three-stage process in which the solids fraction from the primary cellulosic fermentation is further degraded in another reactor while the liquid phase from the primary fermentation is further treated in a third, methanogenic, reactor.
Biorefinery-produced organic acids may be converted into useful fuels by different methods (e.g., those of Holtzapple and of Bradin). Holtzapple et al. (1999) describe processes that produce biofuels of mixed alcohols (MixAlco) and other products such as mixed ketones, by thermochemical treatment of organic acids that are produced by the'action of natural microbial mixtures on biomass material. These processes are described in detail in U.S. Pat. Nos. 5,693,296; 5,865,898; 5,874,263; 5,962,307; 5,986,133; 5,969,189; 6,043,392; 6,262,313; 6,395,926, and 6,478,965. The natural mixed microbial cultures used by Holtzapple are obtained primarily from anaerobic sewage digesters comprising municipal solid waste (MSW) and sewage sludge (SS) that transform chemically pretreated biomass material into volatile fatty acid (VFA) mixtures as described in U.S. Pat. No. 6,043,392 under the “Pretreatment and Fermentation” section. The biomass components that are converted into organic acids are: cellulose, hemicellulose, pectin, sugar, protein, and fats. These processes are characterized by alkaline pretreatment of biomass, followed by the fermentation process, followed by dewatering, thermal conversion to produce ketones, and addition of hydrogen plus catalyst, heat, and pressure, all of which are needed to produce mixed alcohol fuel products. Total reaction processing times are therefore, necessarily long. In order to facilitate VFA accumulation during the fermentation stage, Holtzapple's process typically uses a “stuck” fermentation, in which microbial methane formation is prevented by keeping the pH low and/or by adding specific (toxic) inhibitors of methanogenesis (e.g., bromoform [CHBr3], or iodoform [CHI3]). This causes the fermentation intermediates (organic acids) to accumulate, but also leaves these toxic inhibitors of methanogenesis in the wastewater stream necessitating further cleanup.
Biofuels can also be produced by using specific types of microbes, as opposed to mixed microbial cultures, in order to produce specific types of hydrocarbons exclusively from sugars. For example, Bradin (2007; publication WO 2007/095215 A2) describes a process that produces n-hexane from the fermentation of sugars, using specific natural bacteria or yeast that produce specifically butyric acid as a single product. The butyric acid is then subjected to Kolbe dimerization electrolysis to form n-hexane. However, the preferred microbes are selected to reduce or eliminate acetic acid as a byproduct because it lowers butyric acid yield. Moreover, the preferred microbes must be either naturally isolated or genetically engineered pure cultures, and must be cultivated under controlled conditions to prevent culture contamination, thus reducing flexibility and increasing the cost. In addition, in order to produce sugars from complex carbohydrates such as cellulose and hemicellulose, specific enzymes must be added to the biomass, thus further adding to the reaction processing time and the cost. The Bradin process also requires the separation of lignin from cellulose and hemicellulose, by other enzymes or oxidizing agents to delignify the biomass prior to fermentation. The single n-hexane product also requires further refinement in order to be used as a transportation fuel.
Anaerobic fermentations of plant biomass yield a variety of fermentation end products having high potential energy. Some of these products, like ethanol or butanol, can be recovered by distillation and used directly as motor vehicle fuels. Others, like VFA (e.g., acetic, propionic or butyric acids) can be produced in substantial quantities, but are not directly usable as fuels.
The literature contains a number of examples of conversions of carboxylic acids to hydrocarbons using electrochemistry. For example, the alkyl groups of fatty acids can be combined tail-to-tail during anodic electrolytic decarboxylation to yield alkanes (e.g., ethane from acetic acid, butane from propionic acid, etc.), the so-called Kolbe reaction. The Kolbe reaction can proceed via dimerization of similar radical species to produce single alkanes, or cross-radical reactions with dissimilar radical species to produce alkane mixtures (see Table 1). Moreover, fatty acids can be partially cleaved and converted to alkenes (e.g., ethylene from propionic acid) by the so-called Hofer-Moest reaction. The Hofer-Moest reaction can produce alkenes via deprotonation and alcohols via substitution. Under certain reaction conditions, dienes and trienes can also be produced. The hydrocarbon reaction formulas are shown in Formula 1, comparing one-electron (Kolbe) and two-electron (Hofer-Moest) schemes for electrolytic decarboxylation (Adapted from Lund [2001] and Seebach et al. [1995]).